package com.cg.leetcode;

import org.junit.Test;

import java.util.Arrays;

/**
 * 37.解数独
 *
 * @author cg
 * @program LeetCode->LeetCode_37
 * @create 2022-08-30 21:41
 **/
public class LeetCode_37 {

    @Test
    public void test37() {
        char[][] board = {
                {'5', '3', '.', '.', '7', '.', '.', '.', '.'},
                {'6', '.', '.', '1', '9', '5', '.', '.', '.'},
                {'.', '9', '8', '.', '.', '.', '.', '6', '.'},
                {'8', '.', '.', '.', '6', '.', '.', '.', '3'},
                {'4', '.', '.', '8', '.', '3', '.', '.', '1'},
                {'7', '.', '.', '.', '2', '.', '.', '.', '6'},
                {'.', '6', '.', '.', '.', '.', '2', '8', '.'},
                {'.', '.', '.', '4', '1', '9', '.', '.', '5'},
                {'.', '.', '.', '.', '8', '.', '.', '7', '9'}
        };
        solveSudoku(board);
        System.out.println(Arrays.deepToString(board));
    }

    /**
     * 编写一个程序，通过填充空格来解决数独问题。
     * 数独的解法需 遵循如下规则：
     * 数字 1-9 在每一行只能出现一次。
     * 数字 1-9 在每一列只能出现一次。
     * 数字 1-9 在每一个以粗实线分隔的 3x3 宫内只能出现一次。（请参考示例图）
     * 数独部分空格内已填入了数字，空白格用 '.' 表示。
     * <p>
     * 示例 1：
     * 输入：board = [
     * <p>
     * ["5","3",".",".","7",".",".",".","."],
     * <p>
     * ["6",".",".","1","9","5",".",".","."],
     * <p>
     * [".","9","8",".",".",".",".","6","."],
     * <p>
     * ["8",".",".",".","6",".",".",".","3"],
     * <p>
     * ["4",".",".","8",".","3",".",".","1"],
     * <p>
     * ["7",".",".",".","2",".",".",".","6"],
     * <p>
     * [".","6",".",".",".",".","2","8","."],
     * <p>
     * [".",".",".","4","1","9",".",".","5"],
     * <p>
     * [".",".",".",".","8",".",".","7","9"]
     * <p>
     * ]
     * 输出：[
     * <p>
     * ["5","3","4","6","7","8","9","1","2"],
     * <p>
     * ["6","7","2","1","9","5","3","4","8"],
     * <p>
     * ["1","9","8","3","4","2","5","6","7"],
     * <p>
     * ["8","5","9","7","6","1","4","2","3"],
     * <p>
     * ["4","2","6","8","5","3","7","9","1"],
     * <p>
     * ["7","1","3","9","2","4","8","5","6"],
     * <p>
     * ["9","6","1","5","3","7","2","8","4"],
     * <p>
     * ["2","8","7","4","1","9","6","3","5"],
     * <p>
     * ["3","4","5","2","8","6","1","7","9"]
     * <p>
     * ]
     * <p>
     * 提示：
     * board.length == 9
     * board[i].length == 9
     * board[i][j] 是一位数字或者 '.'
     * 题目数据 保证 输入数独仅有一个解
     *
     * @param board
     */
    public void solveSudoku(char[][] board) {
        helper(board);
    }

    private boolean helper(char[][] board) {
        // 行
        for (int i = 0; i < board.length; i++) {
            // 列
            for (int j = 0; j < board[i].length; j++) {
                if (board[i][j] != '.') {
                    continue;
                }
                // (i,j)位置放置1-9哪个合适
                for (char k = '1'; k <= '9'; k++) {
                    if (isValid(board, i, j, k)) {
                        // 放置
                        board[i][j] = k;
                        // 找到一组立即放回
                        if (helper(board)) {
                            return true;
                        }
                        // 回溯
                        board[i][j] = '.';
                    }
                }
                // 9个数都试完了，都不行，那么就返回false
                return false;
                // 因为如果一行一列确定下来了，这里尝试了9个数都不行，说明这个棋盘找不到解决数独问题的解！
                // 那么会直接返回， 「这也就是为什么没有终止条件也不会永远填不满棋盘而无限递归下去！」
            }
        }
        return true;
    }

    private boolean isValid(char[][] board, int row, int col, char k) {
        // 行
        for (int i = 0; i < 9; i++) {
            if (board[i][col] == k) {
                return false;
            }
        }
        // 列
        for (int i = 0; i < 9; i++) {
            if (board[row][i] == k) {
                return false;
            }
        }
        // 九宫格
        int startRow = (row / 3) * 3;
        int startCol = (col / 3) * 3;
        for (int i = startRow; i < startRow + 3; i++) {
            for (int j = startCol; j < startCol + 3; j++) {
                if (board[i][j] == k) {
                    return false;
                }
            }
        }
        return true;
    }

}
